Recently, portable radio communication devices including a radio communication function have been widely used. The radio communication device can perform radio communication with other radio communication devices by directly transmitting/receiving radio signals to/from other radio communication device, for example. Radio communication by such radio communication device sometimes is referred to as an ad-hoc mode in comparison with an infrastructure mode which requires a base station.
The field intensity of a radio signal transmitted from a radio communication device is known to be inversely proportional to the square or cube of the distance in close proximity to the radio communication device being a signal transmission source and inversely proportional to the distance in no proximity in the ideal space. Such characteristics of the field intensity of a radio signal are established also in the wireless LAN (Local Area Network) specifications such as IEEE 802.11b or 11g, for example. A technique of estimating the distance between radio communication devices with use of such characteristics of the field intensity of a radio signal is described in Patent Literature 1, for example.    Patent Literature 1: JP 2006-300918 (A)